Optimum peel seals are those which may be obtained over a wide sealing temperature range, with minimum variance in the seal value from a control or selected peel strength, and most preferably seals which may be obtained at relatively low sealing temperatures. Thermoplastic films of otherwise desirable attributes for commercial film packages typically do not meet this criteria. Frequently, for example, conventional packaging films exhibit the extremes of a "seal" or "no seal" response to conventional heat sealing techniques. High density polyethylene and many ethylene vinyl acetate copolymers generally fall into this characterization. As such these materials are incapable of providing closely controlled peel strengths except possibly by means of extremely delicate temperature controls which are oftentimes impractical for commercial production.
Ethylene vinyl acetate copolymers have heretofore been utilized, however, in blended form with very high melt index materials such as various waxes, or other ethylene vinyl acetate copolymers of high vinyl acetate content, to provide a sealing layer adapted to the formation of low strength peel seals. Generally, however, unmodified ethylene vinyl acetate copolymers, with desirable concentrations of from about 15 to 35 weight percent vinyl acetate have not been accepted as a good peel seal forming materials, particularly where a peel seal strength of less than about 1 pound per linear inch is desired for the package structure.
Moreover, the addition of such high melt index modifiers does not provide a satisfactory solution in that the modified material is oftentimes very difficult to coextrude in laminar form with other plastics, and also the resultant film may have poor machining characteristics because of the "tacky" or high friction surface typically induced by modifiers of this type.
Accordingly, it would be of advantage if there were available a method for preparing peel seals utilizing heat seal layers blended for improved machining and handling characteristics, efficient coextrudability with various film substrate materials, and which provided close control over the peel strength of the seal at low sealing temperatures, and with sufficient latitude in the effective sealing temperature range to be well adapted for the commercial production of film packages.
It would be of even further advantage to have available such a method wherein packages may be prepared with a combination of peel and high seal strength seals, achieved using a common heat seal layer material, but at distinct sealing temperature ranges, each such sealing temperature range permiting an error of at least about plus or minus 10.degree. F. from the control sealing temperature, to thereby obtain reliable controlled seal strengths without requiring excessively delicate controls over the temperature of the apparatus forming the seals.
It would be of further advantage to the art to have available film packages exhibiting closely controlled peel seal strengths, yet requiring generally unsophisticated sealing temperature controls for their manufacture, and yet additionally packages which may be expediently manufactured exhibiting low strength peel seals, such as along one edge for easy opening, and higher strength, more durable seals along the remaining sealed portions of the package.
These benefits and other cognate advantages are achieved through:
(I) The method of the invention for forming peel seals comprising the steps of utilizing therefor a film comprising at least a two layered laminar structure embodying a substrate layer having intimately adhered to one surface thereof a heat seal layer, the heat seal layer comprising essentially a blend of not less than about 10 and up to about 60 parts by weight of an ethylene homopolymer having a density of between about 0.93 to 0.98 gm/cc, and not less than about 30 and up to about 60 parts by weight of a copolymer of from about 65 to 85 weight percent ethylene and from about 15 to 35 weight percent vinyl acetate, bringing opposed portions of the film into contact under pressure with the heat seal layer of such portions positioned in face to face contact, and applying a sealing temperature to the film above the sealing temperature of the copolymer constituent, but less than the crystalline melting point of the homopolymer constituent to effect a seal strength between said opposed portions of about 1 pound per linear inch in strength or less.
(II) The package of the invention comprising a flexible package fabricated at least in part from a flexible film comprising at least two layers in intimately adhered laminar form, including a substrate layer having intimately adhered to one surface thereof a heat seal layer prepared from a homogeneous blend comprising essentially not less than about 10 and not greater than about 60 parts by weight of an ethylene homopolymer having a density of from between about 0.93 to about 0.98 gms/cc, and not less than about 30 and not greater than about 60 parts by weight of a copolymer of from about 65 to 85 weight percent ethylene and from about 15 to 35 weight percent vinyl acetate, opposed portions of such film being joined by a pressure and heat induced seal utilizing the heat seal layer of each such portion, said seal being characterized by a peel strength in the range from between about 0.1 and about 1.0 pound per linear inch.